The changeup is a unique pitch because it is an off-speed pitch, meaning it is thrown at a slower speed than the fastball. The goal of the changeup is to trick the batter into thinking they are getting a fastball and then actually throwing them a changeup. Ideally, the pitcher is so skilled that their arm looks the same as a fastball as they begin the pitch, but then at the release they throw a changeup.
The pitch is slowed by because the ball is held deeper in the palm and there is less contact with the fingertips and less torque on the ball. This is a great way to trick a batter into thinking there is a fastball coming, but the changeup will hopefully trick the batter into swinging early and missing the ball. If a pitcher throws a fastball at 95 miles per hour, their changeup may clock in at 85 miles per hour.
There are two different basic changeup pitches, the three finger changeup and the circle changeup.